Field of the Invention
The present disclosure relates to a high-order optical fiber multi-wavelength filter, and a continuous wavelength tuning method using the same, and more particularly, to a high-order optical fiber multi-wavelength filter having two or more birefringent elements (BEs), for continuously tuning the wavelengths of the transmission spectrum of the filter by controlling the azimuth angle of a wave plate, and a continuous wavelength tuning method using the same.
Discussion of the Related Art
Owing to its simple structure, use easiness, and excellent optical fiber compatibility, an optical fiber multi-wavelength filter has been considered as a useful wavelength selection element available for processing an optical signal, and blocking an unnecessary signal causing crosstalk in an optical network.
The optical fiber multi-wavelength filter is applicable to a multi-wavelength optical fiber laser, a microwave photonic filter, and generation of an optical pulse train. The wavelength conversion function of the optical fiber multi-wavelength filter may be useful in selecting an intended wavelength component or preventing interference between wavelength components in a multi-wavelength light source. Research has actively been conducted on a Sagnac birefringence loop (SBL), a Mach-Zehnder interferometer (MZI), a Lyot birefringent filter, a polarization-diversity loop configuration (PDLC), and so on in order to implement a multi-wavelength filter having wavelength variability.
Among them, a PDLC-based multi-wavelength filter configured with a polarization beam splitter (PBS) is much advantageous over an SBL-based multi-wavelength filter in terms of wavelength switching and tuning efficiency of a pass band. Particularly, the PDLC-based multi-wavelength filter is robust against disturbances such as temperature and vibration, compared to an MZI-based filter. Further, unlike the Lyot birefringent filter, the PDLC-based multi-wavelength filter has a transmission spectrum which is not dependent on an input polarization.
In the case of a PDLC-based zero-order multi-wavelength filter using one polarization-maintaining fiber (PMF) as a birefringent element (BE), wavelength tuning of a transmission spectrum may be performed through various combinations of half-wave plates and quarter-wave plates.
Further, transmission characteristics such as a flat-top band or narrowband spectrum may be achieved through a Solc-type PDLC-based first-order multi-wavelength filter in which two PMFs of the same length are concatenated with an angle offset of 45° between their principal axes, and a Lyot-type PDLC-based first-order multi-wavelength filter in which two PMF loops with one PMF loop twice longer than the other PMF loop are concatenated with an angle offset of 60° between their principal axes. Interleaving of flat-top band and narrowband transmission spectrums, that is, half-period switching of a multi-wavelength spectrum may be performed by controlling an angle difference between the principal axes of two PMFs by means of half-wave plates in this PDLC-based first-order multi-wavelength filter.
In general, a high-order optical fiber multi-wavelength filter having two or more BEs may be useful for efficient processing of an optical signal in an optical network system. However, it is difficult to achieve a continuous wavelength tuning function with the high-order optical fiber multi-wavelength filter having two or more BEs.
This is because it is not easy to detect an appropriate combination of wave plates and BEs (i.e., PMFs), and it is very complicated to determine the azimuth angle of each individual optical device.
Therefore, if a high-order optical fiber multi-wavelength filter having two or more BEs is configured by appropriately selecting the types and number of wave plates, and positions of the wave plates relative to BEs, and a wavelength tuning technique for continuous tuning of the wavelengths of a spectrum in the high-order optical fiber multi-wavelength filter is derived, it is expected that this will significantly increase the optical application range and application efficiency of the high-order optical fiber multi-wavelength filter.